The present invention relates generally to the emission of light by semiconductors. More particularly, the present invention relates to a method and apparatus for obtaining light emission from indirect bandgap semiconductor materials, such as crystalline silicon, the apparatus being compatible with and capable of being formed by known VLSI-process technologies.
An LED is a two-terminal (p-n junction), rectifying electronic device which, when forward biased, causes electrons and holes to recombine and in so doing emit light. When the LED is fabricated within a semiconductor the electrons are supplied to the p-n junction region from the n-type region and the holes are supplied from the p-type region. The energy of the emitted light (and hence its wavelength) is equal to the difference in energies of the two recombining carriers. For semiconductors, the energy difference is usually very nearly equal to the bandgap energy. For many semiconductors, most notably gallium arsenide (GaAs), the band gap is "direct" which means that the electron and hole can recombine by simply emitting a photon to carry off the energy difference. For other semiconductors, such as silicon (Si), the band gap is "indirect" which means that a phonon or lattice vibration must be excited in the process of light emission. The consequence is that indirect gap materials are a thousand to a million times less efficient at light emission (i.e., less light and more heat are emitted from the electron-hole recombination) than direct gap materials. For this reason, the predominant solid state LEDs seen today are fabricated from direct gap materials such as GaAs and its alloys.
There are many examples of semiconductors which have, to date, not been practical for use as light emitters because of their indirect bandgap. Examples include silicon, germanium, AlGaAs for high Al concentrations, etc. Silicon is a very important specific case of a semiconducting material limited by its indirect gap. Many novel technologies would be enabled if Si could be made to emit light efficiently. Functional integration of optoelectronic capabilities with the digital and analog processing capabilities of Si would allow monolithic fabrication of high speed, wireless interconnections on, or between, chips and/or boards, high bandwidth, wireless local area networking, light-induced marking methods, etc.
Various schemes have been pursued to effectively convert the indirect gap to a direct gap in Si and its alloys. One such approach utilizes band folding generated by a superlattice of indirect gap materials. See U. Gnutzmann et al., Theory of Direct Optical Transitions in an Optical Indirect Semiconductor with a Superlattice Structure, Appl. Phys., vol. 3, p. 9 (1974). Although this method has apparently worked to produce a direct gap, the resulting material has a very small cross section for radiative recombination. See, e.g., M. Hybertsen et al., Theory of Optical Transitions in S/Ge(001) Strained-Layer Superlattices, Phys. Rev. B, vol. 36, no. 18, p. 9683 (1987). The excited carriers therefore have time to find other recombination channels which are generally nonradiative. The important device criterion of high efficiency, namely a high ratio of number of photons emitted per carrier-pair recombination, is therefore not met.
Another approach has been to incorporate impurities into the indirect gap material to localize electron-hole recombination at the impurity atoms. The wavefunctions of the carriers are thereby modified, resulting in direct-gap type recombinations. Two examples of this are Nitrogen in GaP (D. G. Thomas et al., Isoelectronic Traps Due to Nitrogen in Gallium Phosphide, Phys. Rev., vol. 150, no. 2, p. 680 (1966)), and Erbium in Si (H. Ennen et al., 1.54 .mu.m Electroluminescence of Erbium-Doped Silicon Grown by Molecular Beam Epitaxy, Appl. Phys. Lett., vol. 46, no. 4, p. 381 (1985)). The former case has thus far not been generalized to other semiconductor materials. For example, in the latter case, the maximum recombination efficiency is very small and the impurities are ineffective for radiative recombination at room temperature. Again, the efficiency criterion fails.
Much effort has also been expended to join direct gap materials with silicon to achieve the integration. Attempts have included trying to grow GaAs directly on Si. See, e.g., Akiyama et al., Growth of GaAs on Si and its Applications to FETs and LEDs, Mat. Res. Soc. Symp. Proc. 67, p. 53 (1986). This has had only limited success to date. Also, hybrid techniques have been developed such as bonding GaAs devices to Si wafers (as disclosed by Yablonovich et al., in Van der Waals Bonding of GaAs Epitaxial Liftoff Films Onto Arbitrary Substrates, Appl. Phys. Lett., vol. 56, no. 24, p. 2419 (1990)). Both of these integrative techniques introduce complications into the Si device processing because new steps are required which are not part of standard Si device fabrication methods.
It is believed that one method for converting an indirect gap semiconductor to a direct gap semiconductor is to localize the wavefunctions of the carriers in the indirect gap material. If a carrier is confined, quantization of the carrier's wavefunction occurs in the dimension of confinement, thereby restricting the energy of the carrier to discrete values E.sub.n with n=1,2,3, . . . (See, for example, Holonyak et al., Quantum-Well Heterostructure Lasers, IEEE J. Quant. Elect., vol. QE-16, no. 2, p. 170, (1980)). These levels ("quantum levels") are typically illustrated by a bound-state diagram such as that shown in FIG. 1 for a SiO.sub.2 /Si quantum well structure. Various studies have demonstrated that as the thickness of the semiconductor layers in which the carriers are confined approaches the carrier de Broglie wavelength (.lambda.=h/p, where h is Planck's constant and p is carrier momentum) the separation between levels becomes significant (e.g., greater than the thermal energy of the carriers--25 mV at room temp.) Associated with this quantum confinement of carriers is a shifting of photoluminescence (PL) peak energies to higher values (shorter wavelengths) and increased efficiency of bandedge recombination. Quantum well, quantum wire and quantum dot structures are canonical examples of this technique.
Consider a very simple but rather accurate approximation as a useful model of quantum confinement effects. For a d-dimensional infinitely deep, rectangular potential well, the energy levels are given by ##EQU1## where .h slashed. is Planck's constant divided by 2.pi., n.sub.i, the quantum number, is an integer greater than or equal to 1 (equal to the number of half waves the particle wavefunction has packed between the confining walls), m.sub.i is the carrier effective mass, and L.sub.i is the well width for the ith dimension. The lowest energy occurs for all n.sub.i =1. Thus in "d" dimensions, for an isotropic mass and all L.sub.i =L (square well), E=d.pi..sup.2 .h slashed..sup.2 /2mL.sup.2. As shown in FIG. 1, the energy of an electron in a quantum confined semiconductor will thus be increased by an amount .DELTA.E.sub.e.sbsb.1, .DELTA.E.sub.e.sbsb.2, etc., and the energy of a hole will thus be increased by an amount .DELTA.E.sub.h.sbsb.1, .DELTA.E.sub.h.sbsb.2, etc. (where the masses are generally different).
The photon emitted from electron-hole recombination has total energy given by EQU E=E.sub.g +.DELTA.E.sub.e.sbsb.n, +.DELTA.E.sub.h.sbsb.n ( 2)
For d-dimensional wells assuming for simplicity that m.sub.e =m.sub.h =m.sub.free electron, the increase .DELTA.E=.DELTA.E.sub.e.sbsb.n +.DELTA.E.sub.h.sbsb.n over E.sub.g is.perspectiveto.d/L.sup.2 in [eV] for L in [nm] and is shown in FIG. 2 for d=1,2and3.
Quantum confinement of the carriers also results in a break in the translational invariance of the Si network (i.e., the periodic structure is made more "uncertain") and leads to a breakdown of the bulk momentum selection rules. Thus, phonon generation in the recombination of carriers becomes unnecessary for the generation of photons (i.e., the number of direct transitions increases while the number of indirect transitions decreases) and the efficiency of the device is increased. Moreover, spatial confinement of the electrons and holes in the same small volume increases the wavefunction overlap and thereby the radiative recombination probability.
Examples of electron confinement are now numerous. Several examples exist for crystalline and amorphous silicon, for example, D. J. DiMaria et al., Electroluminescence Studies in Silicon Dioxide Films Containing Tiny Silicon Islands, J. Appl. Phys. 56, no. 2, p. 401 (1984), and S. Furukawa et al., Three Dimensional Quantum Well Effects in Ultrafine Silicon Particles, Japanese J. Appl. Phys., vol. 27, no. 11, p. L2207 (1988). The most recent work has been on p-type Si formed into small but connected clumps by electrochemical etching (so called "porous silicon" as described for example by M. I. J. Beale et al., in Microstructure and Formation Mechanism of Porous Silicon, Appl. Phys. Lett., vol. 46, no. 1, p. 86 (1985)). This porous silicon is formed by immersing a silicon wafer in an acidic electrochemical bath which bores small holes into the wafer. The wafer is then chemically etched to enlarge the holes. This leaves a somewhat random pattern of connected silicon threads, resulting in a sponge-like structure. When cross-sections of the connected Si threads are on the order of 3 nm, visible and reasonably efficient room temperature photoluminescence is observed as demonstrated by L. T. Canham in Silicon Quantum Wire Array Fabricated by Electrochemical and Chemical Dissolution of Wafers, Appl. Phys. Lett. 57, no. 10, p. 1046 (1990).
Electroluminescence has been observed in 1- and 2-d confined structures in direct gap materials. Electroluminescence has also been induced by tunneling injection into Si precipitates within silicon dioxide as demonstrated by D. J. DiMaria et al., in Electroluminescence Studies in Silicon Dioxide Films Containing Tiny Silicon Islands, J. Appl. Phys. 56, no. 2, p. 401(1984). However, to date it has not been possible to fabricate an electroluminescent device (for example an LED) in Si, or any other indirect gap material, with useful performance characteristics--particularly electro-optical efficiency. It is one purpose of the present invention to provide such devices.
In addition, semiconductor LEDs primarily emit light at a single wavelength, which is principally determined by the semiconductor material from which the LED is formed. Often, the output wavelength of the LEDs must be "tuned" or switched between two or more different output wavelengths to optimize the applicability of the LED. External apparatus such as diffraction gratings, holograms, and the like, have been employed to tune or switch the output wavelength(s) of light emitting devices. Such external tuning arrangements are disadvantaged, though, since they increase the size and complexity of the light source. It is therefore preferable to be able to tune the device by controlling the characteristics of the device itself, and thus it is among the other purposes of the present invention to facilitate improved tunability or switching of the output wavelength of the light emitting device.